1. Technical Field
The present invention relates to a probe card configuration for a test system used to test integrated circuits (ICs) on a wafer. More particularly, the present invention relates to a probe card configuration with intelligent on board features that can, for example, enable the probe card to distribute a single channel from a test system controller to multiple test probes to connect to the ICs on a wafer.
2. Related Art
When testing ICs on a wafer, it is cost effective to test as many devices as possible in parallel, thus reducing the test time per wafer. Test system controllers have evolved to increase the number of channels and hence the number of devices that can be tested in parallel. However, a test system controller with increased test channels is a significant cost factor for a test system, as is a probe card with complex routing lines used to accommodate multiple parallel test channels. It is, thus, desirable to provide an overall probe card architecture that allows increased test parallelism without requiring increased test system controller channels and without increased probe card routing complexity.
With limited test system controller resources, fanning out a signal from a test system controller in the probe card to multiple transmission lines may be desirable, since the increased cost of probe card routing complexity will typically be outweighed by the cost of a new test system controller. A test system controller has resources to enable testing a fixed number of Devices Under Test (DUTs) on a wafer. With advancing technology, more DUTs are fabricated on a single wafer. To avoid the cost of a new test system controller, either multiple touchdowns of a test system to the wafer are performed, or the test signals normally provided to a single DUT are fanned out to multiple DUTs in the probe card. The later may be more desirable for burn in testing where during heating of the wafer, multiple touch downs of the probe card to the wafer is sometimes impractical. Further, less touch downs to the wafer decrease the likelihood of damaging the wafer, and less touch downs limit wear on the probes in the test system, which may be expensive to replace.
Fan out of the test signals in a probe card between a test system controller and DUTs, however, not only increases the complexity of a system, but also can cause inaccurate test results. To better assure test integrity, increased circuitry can be provided on the probe card to minimize the effect of a fault on one of the fan out lines. With a test system having probe card fan out, a fault (short circuit) in a component connected on a fanned out line will severely attenuate the test signal for all devices on the fanned out test system channels. U.S. Pat. No. 6,603,323 entitled “Closed-Grid Bus Architecture For Wafer Interconnect Structure,” incorporated herein by reference, describes a solution by providing isolation resistors between the channel line branch points and probes to reduce attenuation caused by the faulty component. A further solution is provided in U.S. patent application Ser. No. 10/693,133, incorporated herein by reference, entitled “Isolation Buffers With Controlled Equal Time Delays” describing a system where isolation buffers are used between channel line branch points and probes, with circuitry included to assure the isolation buffers each provide a uniform delay. Other problems, however, may occur with the added circuitry affecting test integrity, as recognized in development of the present invention.
With the cost of test system controller systems making their long term retention desirable, probe cards are desirable that can further take on expanded test system functions to increase the lifecycle of an outdated test system. Probe cards, serving as an interface between a test system controller and a wafer, are typically much less expensive than a test system controller, and typically replaced after a much shorter lifecycle than the test system controller due to wear of probes on the probe card.
FIG. 1 shows a block diagram of a test system using a probe card for testing DUTs on a semiconductor wafer. The test system includes a test system controller 4, or general purpose computer, connected by a communication cable 6 to a test head 8. The test system further includes a prober 10 made up of a stage 12 for mounting a wafer 14 being tested, the stage 12 being movable to contact the wafer 14 with probes 16 on a probe card 18. The prober 10 includes the probe card 18 supporting probes 16 which contact DUTs formed on the wafer 14.
In the test system, test data is generated by the test system controller 4 and transmitted through the communication cable 6, test head 8, probe card 18, probes 16 and ultimately to DUTs on the wafer 14. Test results are then provided from DUTs on the wafer back through the probe card 18 to the test head 8 for transmission back to the test system controller 4. Once testing is complete, the wafer is diced up to separate the DUTs.
Test data provided from the test system controller 4 is divided into the individual test channels provided through the cable 6 and separated in the test head 8 so that each channel is carried to a separate one of the probes 16. The channels from the test head 8 are linked by flexible cable connectors 24 to the probe card 18. The probe card 18 then links each channel to a separate one of the probes 16.
FIG. 2 shows a cross sectional view of components of a typical probe card 18. The probe card 18 is configured to provide both electrical pathways and mechanical support for the spring probes 16 that will directly contact the wafer. The probe card electrical pathways are provided through a printed circuit board (PCB) 30, an interposer 32, and a space transformer 34. Test data from the test head 8 is provided through flexible cable connectors 24 typically connected around the periphery of the PCB 30. Channel transmission lines 40 distribute signals from the connectors 24 horizontally in the PCB 30 to contact pads on the PCB 30 to match the routing pitch of pads on the space transformer 34. The interposer 32 includes a substrate 42 with spring probe electrical contacts 44 disposed on both sides. The interposer 32 electrically connects individual pads on the PCB 30 to pads forming a land grid array (LGA) on the space transformer 34. Traces 46 in a substrate 45 of the space transformer 34 distribute or “space transform” connections from the LGA to spring probes 16 configured in an array. The space transformer substrate 45 is typically constructed from either multi-layered ceramic or organic based laminates. The space transformer substrate 45 with embedded circuitry, probes and LGA is referred to as a probe head.
Mechanical support for the electrical components is provided by a back plate 50, bracket (Probe Head Bracket) 52, frame (Probe Head Stiffener Frame) 54, leaf springs 56, and leveling pins 62. The back plate 50 is provided on one side of the PCB 30, while the bracket 52 is provided on the other side and attached by screws 59. The leaf springs 56 are attached by screws 58 to the bracket 52. The leaf springs 56 extend to movably hold the frame 54 within the interior walls of the bracket 52. The frame 54 then includes horizontal extensions 60 for supporting the space transformer 34 within its interior walls. The frame 54 surrounds the probe head and maintains a close tolerance to the bracket 52 such that lateral motion is limited.
Leveling pins 62 complete the mechanical support for the electrical elements and provide for leveling of the space transformer 34. The leveling pins 62 are adjusted so that brass spheres 66 provide a point contact with the space transformer 34. The spheres 66 contact outside the periphery of the LGA of the space transformer 34 to maintain isolation from electrical components. Leveling of the substrate is accomplished by precise adjustment of these spheres through the use of advancing screws, or leveling pins 62. The leveling pins 62 are screwed through supports 65 in the back plate 50 and PCB 30. Motion of the leveling pin screws 62 is opposed by leaf springs 56 so that spheres 66 are kept in contact with the space transformer 34.
FIG. 3 shows an exploded assembly view of components of the probe card of FIG. 2. FIG. 3 shows attachment of the back plate 50, PCB 30, and bracket 52 using two screws 59. Four leveling screws 62, are provided through the back plate 50 and PCB 30 to contact four spheres 66 near the corners of the space transformer substrate 34. The frame 54 is provided directly over the space transformer substrate 34, the frame 54 fitting inside the bracket 52. The leaf springs 56 are attached by screws 58 to the bracket 52. Two screws 58 are shown for reference, although additional screws 58 (not shown) are provided around the entire periphery to attach the leaf springs.
FIG. 4 shows a perspective view of the opposing side of PCB 30 illustrating the arrangement of connectors 24 around its periphery. In FIG. 3, the connectors 24 of the PCB 30 are facing down and not shown. In typical probe cards, the connectors 24 (typically zero insertion force (ZIF) connectors) provide flexible cable connections located around the periphery of the probe card, and are configured to mate with connectors that are typically arranged in a similar fashion on the test head. Although illustrated as ZIF connectors, other connector types may be used, such as pogo pins, non-ZIF flexible cable connectors, conductive elastomer bumps, stamped and formed spring elements, etc.